Cold marine sediments harbor endospores of fermentative and sulfate-reducing, thermophilic bacteria. These dormant populations of endospores are believed to accumulate in the seabed via passive dispersal by ocean currents followed by sedimentation from the water column. However, the magnitude of this process is poorly understood because the endospores present in seawater were so far not identified, and only the abundance of thermophilic sulfate-reducing endospores in the seabed has been quantified. We investigated the distribution of thermophilic fermentative endospores (TFEs) in water column and sediment of Aarhus Bay, Denmark, to test the role of suspended dispersal and determine the rate of endospore deposition and the endospore abundance in the sediment. We furthermore aimed to determine the time course of reactivation of the germinating TFEs. TFEs were induced to germinate and grow by incubating pasteurized sediment and water samples anaerobically at 50°C. We observed a sudden release of the endospore component dipicolinic acid immediately upon incubation suggesting fast endospore reactivation in response to heating. Volatile fatty acids (VFAs) and H2 began to accumulate exponentially after 3.5 h of incubation showing that reactivation was followed by a short phase of outgrowth before germinated cells began to divide. Thermophilic fermenters were mainly present in the sediment as endospores because the rate of VFA accumulation was identical in pasteurized and non-pasteurized samples. Germinating TFEs were identified taxonomically by reverse transcription, PCR amplification and sequencing of 16S rRNA. The water column and sediment shared the same phylotypes, thereby confirming the potential for seawater dispersal. The abundance of TFEs was estimated by most probable number enumeration, rates of VFA production, and released amounts of dipicolinic acid during germination. The surface sediment contained ∼105–106 inducible TFEs cm-3. TFEs thus outnumber thermophilic sulfate-reducing endospores by an order of magnitude. The abundance of cultivable TFEs decreased exponentially with sediment depth with a half-life of 350 years. We estimate that 6 × 109 anaerobic thermophilic endospores are deposited on the seafloor per m2 per year in Aarhus Bay, and that these thermophiles represent >10% of the total endospore community in the surface sediment.

Endospores of anaerobic thermophilic bacteria (thermospores) are ubiquitous in cold marine sediments. These misplaced thermophiles likely originate from warm environments and are delivered to the seafloor by passive dispersal through the water column. The few studies of the abundance of thermospores that exist have only quantified a subset of the anaerobic metabolic types possibly present and the data density has been too low to address patterns of dispersal. Here, we introduce isothermal microcalorimetry as a fast tool to quantify the abundance of thermospores in environmental samples. We developed and tested the method on samples from the water column and sediment of Aarhus Bay, Denmark. The thermospores were stimulated to germinate and grow by heating the samples to 50°C inside an isothermal microcalorimeter and the metabolic heat production was followed across the exponential growth phase. The number of germinated thermospores was then calculated by dividing the total heat production rate by the cell specific heat production rate, empirically determined in parallel experiments. The abundance of thermospores in the water column of Aarhus Bay was 1.2 × 103−3.6 × 103 cells × mL−1 seawater. The abundance of thermospores in sediment in Aarhus Bay was 1.1 × 106−6.1 × 106 cells × mL−1 sediment and their half‐life was ∼70 yr. As application example we used isothermal microcalorimetry to test if thermospores were aggregated at a pockmark in Limfjorden (Denmark) with seepage of thermogenic gas and oil.

Organism body size has been demonstrated to be a discriminating element in shaping the response of living beings to environmental factors, thus playing a fundamental role in community structuring. Despite the importance of studies elucidating relations among communities of different size levels in ecosystems, the attempts that have been made in this sense are still very scarce and a reliable approach for these research still has to be defined. We characterized the benthic communities of bacteria, microbial eukaryotes, meiofauna and macrofauna in a coastal environment, encompassing a 10000-fold gradient in body size, testing and discussing a mixed approach of molecular fingerprinting for microbes and morphological observations for meio- and macrofauna. We found no correlation among structures of the different size-level communities: this suggests that community composition at one size-level could have no (or very low) influence on the community composition at other size-levels. Moreover, each community responds in a different way to the environmental parameters and with a degree of sensitivity which seems to increase with organism size. Therefore, our data indicate that the characterization of all the different size levels is clearly a necessity in order to study the dynamics really acting in a system.

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